Glycolide is useful as a starting material for production of polyglycolic acid which is a resin material excellent in hydrolyzability in living things or in the earth and also excellent in gas-barrier property. More specifically, polyglycolic acid is a polymer having a recurring unit of —(—CH2—CO—O—)— obtained by dehydro-polycondensation of glycolic acid (i.e., α-hydroxyacetic acid), but according to this process, it is difficult to obtain polyglycolic acid of a high molecular weight. Accordingly, a process of ring-opening polymerization of glycolide (CH2—CO—O)2 which is a cyclic dimer ester of glycolic acid to obtain a high-molecular weight polyglycolic acid, is advantageous (Patent document 1 listed below).
In order to produce polyglycolic acid from glycolide as a starting monomer on a commercial scale, it is indispensable to supply high-purity glycolide economically. Glycolide is a cyclic ester formed by elimination of two water molecules from two molecules of glycolic acid, but esterification of glycolic acid generally results in oligomers and fails to provide glycolide. Instead thereof, a process of de-polymerizing the thus-formed glycolic acid oligomer to produce glycolide is well known.
However, commercially produced glycolic acid oligomer contains various impurities which are presumably principally attributable to glycolic acid production process, and these impurities are known to exert adverse effects to the depolymerization of glycolic acid for producing glycolide.
For example, our research group has clarified that hydrolysis of glycolic acid oligomer formed by polycondensation (esterification) of commercially available glycolic acid aqueous solution results in glycolic acid accompanied with a small amount of impurity carboxylic acids not having hydroxyl group, such as diglycolic acid, methoxyacetic acid and oxalic acid. Herein, “impurity carboxylic acids” mean carboxylic acids regarded as impurities of glycolic acid (which are likely to accompany glycolic acid but are not convertible into glycolide). Based on the knowledge, our research group has developed a glycolide production process wherein a starting glycolic acid oligomer with a reduced amount of such impurity carboxylic acids is used in order to exclude adverse effects thereof to glycolide production (Patent document 2 below), and a glycolide production process wherein an alcoholic hydroxyl group-containing compound is caused to be co-present in the depolymerization system in order to reduce adverse effects of such impurity carboxylic acids during the depolymerization (Patent document 3 below).
However, commercially produced glycolic acid oligomer further contains various impurities principally attributable to the glycolic acid production process in addition to the above-mentioned impurity carboxylic acids.
For example, the above-mentioned impurity carboxylic acids, such as diglycolic acid, methoxyacetic acid and oxalic acid are impurities generally contained in glycolic acid obtained by carboxylation of formalin with carbon dioxide in the presence of an acid catalyst (the high-pressure process, e.g., Patent document 4 below), but the process of purification thereof by using an ion exchange resin (“IRA-68”, as disclosed in Patent document 5 below) can result in contamination with a nitrogen-containing substance such as amino group. Indeed, nitrogen-containing substances have been detected in commercially available glycolic acid aqueous solution.
Further, according to a glycolic acid production process by hydrolysis of glycolonitrile known to result in less impurity carboxylic acids (e.g., Patent document 6 below) can result in contamination with more nitrogen-containing substances in the product glycolic acid.
Accordingly, glycolic acid oligomer obtained by polycondensation of glycolic acid obtained in the above-described manner can contain nitrogen-containing substances as described above in addition to the impurity carboxylic acids.
However, the effect of such nitrogen-containing substances in glycolide production by depolymerization of glycolic acid oligomer has not been known.
Patent document 1: WO2005/044894A1
Patent document 2: JP2002-114775A
Patent document 4: JP2004-523596A
Patent document 3: U.S. Pat. No. 2,152,852
Patent document 5: U.S. Pat. No. 3,859,349
Patent document 6: JP62-267257A
Patent document 7: WO2006/064611A1